Method of forming a refractory metal body containing dispersed refractory metal carbides



United States Patent METHOD OF FORMING A REFRACTORY METAL BODY CONTAINING DISPERSED REFRACTORY METAL CARBIDES Eugene J. Delgrosso, Walling'ford, and Leonard A. Friedrich, West Hartford, Conn, assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware No Drawing. Filed Apr. 21, 1966, Ser. No. 545,794

6 Claims. (Cl. 75-203) ABSTRACT OF THE DISCLOSURE A process for forming dispersion strengthened alloy articles of the refractory metals such as columbium, tantalum, molybdenum, tungsten, titanium and alloys thereof in which fine powders of the desired metal or metals are provided with a thin carbide shell in a fluidized bed and are subsequently warm or hot worked to fracture the shell and uniformly disperse the carbide throughout the article.

The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

This invention relates to the production of dispersion strengthened alloys and dispersion strengthened alloy articles. It contemplates the surface alloying of fine powders to effect the formation of a hard phase envelope around each of the powder particles which can subsequently be warm or hot worked in such a manner that the hard phase envelopes are broken up and uniformly distributed throughout the article to function as a dispersoid phase imparting good elevated temperature strength and creep resistance to the alloy matrix. The procedure hereinafter described offers particular promise for those alloy systems which exhibit limited carbon solubility and have high melting points and, accordingly, it is particularly applicable to those alloys wherein the base metal is columbium, tantalum, molybdenum, tungsten or titanium.

It is extremely difficult to satisfactorily alloy the abovementioned metals with carbon, for example, directly from a melt containing the requisite components in the desired proportions or from a'similar mixture by powder metallurgy techniques. The physical properties of the resultant alloy are not only dependent upon the total amount of carbon present within the alloy but also upon its distribution within the matrix metal. Theoretically, the strength of the alloy incorporating the hard dispersoid phase, a carbide phase for example, is a function of the volume fraction of the carbide, the dispersant particle size and the distribution within the alloy. The conventional techniques of alloy preparation do not satisfactorily provide the desired distribution and small particle size because of carbon agglomeration. As a result the alloy does not usually approach its theoretical strength but, even more importantly, because of the non-uniformity of distribution of the dispersoid phase, the physical properties of the resulting alloy are not reproducible or predictable even when essentially identical preparation techniques are utilized.

It is a primary object of the instant invention to provide a method of producing dispersion strengthened alloys of the higher melting point metals in a manner wherein the dispersion strengthener is uniformly distributed throughout the matrix structure and wherein the resulting alloy is entirely reproducible as to physical properties.

For the sake of brevity, the following discussion has been directed primarily to the formation of carbides on the columbium base alloys. However, it will be understood that the nature of this technique is such that it may be used in connection with any elemental metal capable of forming oxides, nitrides, borides and the like, as well as carbides.

In this process a thin envelope or film of the desired dispersion strengthener is formed on thesurface of the individual particles of fine powders of the matrix metal by a chemical reaction or surface alloying mechanism, the individual particles each reacting relatively uniformly with the reactant used to produce the envelope. Preferably, the desired reaction is effected in a gas reactor wherein the fine powders are held fluid in an inert gas stream at an elevated temperature and a reactant gas is admixed with the inert gas stream. Accordingly, the individual particles are afforded the maximum exposure to the reactant gas and, at the same time, the problem of agglomeration or particle-to-particle sintering, which might occur if the particles were not held in agitated suspension, is avoided, until the desired hard phase film has been formed. The reactant gas is admitted to the reactor as a small percentage of the total gas flow to prevent the buildup of an elemental ingredient, as for example unreacted carbon, in the case that is chemically formed on the individual particles. The particular gas flow rates are dependent to some extent upon the materials to be alloyed and further upon the temperature at which the reactor is held. It is contemplated, however, that the volume fraction of the reactant gas will be sufficiently small to permit the desired surface alloying to proceed without the entrapment or buildup of unalloyed elements.

In addition to the hereinafter described columbium- Zirconium alloy systems, the techniques described are similarly applicable to the following alloys, cited by way of example: columbium-titanium, molybdenum-titanium, molybdenum titanium zirconium, tantalum-zirconium, tantalum-hafnium, tantalum-zirconium-hafnium, tungstenzirconium, tungsten-hafnium and titanium-aluminum. As previously indicated, this technique oifers particular promise for those elements such as tantalum, tungsten and molybdenum which exhibit quite limited carbon solubility and have very high melting points. Further, since many of the tungsten and molybdenum alloys are prepared primarily by powder metallurgy processes, the technique described herein may be readily incorporated into existing processes without any significant disruption of the normal metallurgical practices.

Having produced the desired alloyed powder particles, including the thin envelope comprising the dispersion strengthener, the envelopes are then broken up by a warm or hot working process such as extrusion. The working process fractures the thin envelope surrounding each particle and the fractured pieces are uniformly dispersed through the wrought alloy matrix and result in strengthening due to: (1) Classical fine particle dispersoid distribution, (2) pinning of tangles of dislocation and dislocation subcells, and (3) the development of the fragmented particle skeletal structure in the wrought material.

In this process, articles of density have been achieved without the necessity of a sintering operation. Since the hard phase is produced only as a thin shell around the particles and the matrix has not been reacted and remains perfectly ductile, a high degree of cold fabricability is maintained. This is particularly true since it will be realized that the dispersion strengthener normally comprises a very small percentage of the alloy composition as a whole, the bulk of the matrix metal remaining unreacted and, hence, ductile.

Because of the close control which is possible over the formation of the thin envelopes both as to composition and as to thickness, and the further close control over the working process, the distribution of the hard phase throughout the alloy matrix is consistently uniform and hence the process is entirely reproducible. As a result, the finished wrought alloys have completely predictable metallurgical and physical properties and the articles produced in one batch are identical to those produced in subsequent batches as far as these properties are concerned.

The initial stages of the process described envision the formation of fine powders of the matrix metal. It is understood that, in the case of some alloys, special techniques are required to permit the requisite powdering, dependent generally on the overall ductility of the alloy. Frequently, the metals are embrittled with hydrogen to achieve this end, the hydrided powders subsequently being subjected to a dehydriding process to remove this impurity and, incidentally, to expose a generally highly active metal surface. Further, it will be understood that the exposure of a clean, active metal surface on each particle is desirable in most cases although not always essential. For this reason, it will usually be advantageous to maintain the powders in a non-contaminating atmosphere after the impurities have been removed and prior to the formation of the hard phase envelope.

In the most preferred process the formation of powders of (-200 to 325) mesh are utilized. It should be pointed out, however, that in this process, as distinct from other powder processes wherein a dispersion strengthener is deposited on each particle and not chemically formed thereon, the control of particle size is not particularly critical.

The powders are fluidized in an inert gas stream, such as argon, in a gas reactor and brought to a temperature of 1200 F. to 2000 F. depending upon the material being processed. Any gas which is non-reactive with the individual components may be used for this purpose although one of the noble gases is preferred. A reactant gas, such as methane in the production of the carbide dispersoid phase, is admitted to the reactor usually admixed with the argon stream, the volume flow rate of the reactant gas constituting a very small percentage of the total gas flow. In the carburization of the columbium base alloys the ratio of methane to argon is approximately 1/40. The reaction parameters are maintained for a period of from to 24 hours depending on the parameters established, the materials being processed and the desired carbide buildup, to produce a thin envelope on the individual particles.

The particular fluidized bed apparatus utilized in the test work described comprised a 1 /2 inch diameter Vycor reactor with a V-column distributor utilizing glass wool as the diffuser medium. In this apparatus the inert gas flow rates were generally maintained between 2000 and 400 cc./min. with corresponding reactant gas flow rates of 65-100 cc./min. As previously discussed, the flow rate of the inert gas was such as to insure proper fiuidization of the powders and the methane flow rates were adjusted to afford the desired phase buildup on the individual particles to the desired thickness Without the inclusion of any unreacted elemental components such as free carbon. To insure uniformity of the product close adherence to the operating parameters of the process was maintained from batch to batch.

It is, of course, understood that the particular dispersion strengthener formed in a given instance is dependent upon the composition of the metal being processed as well as upon the nature of the reactants being utilized and the operating conditions of the apparatus. In the formation of the carbide dispersion strengthener, it is possible to form subcarbides, monocarbides and dicarbides as well as complex carbides and mixtures and, accordingly, similar species of the nitrides, oxides and borides. All of these compositions are intended to be included Within the term dispersion strengthener as used herein. It should additionally be pointed out that it is not necessarily the major component of the alloy which reacts with the reactant gas to form the dispersion strengthener. In the carburization of columbium-zirconium alloys, for example, the zirconium carbide is formed along with complex columbium-zirconium carbides.

As used herein, the term matrix metal means the alloy composition as it exists prior to the formation of the hard phase envelope.

It is possible to achieve a degree of dispersion strengthening when the formation of the envelope around the individual particles is incomplete and, thus, does not encompass the entire surface of the particle. This is very apt to be the case when very minor amounts of dispersoid are desired in the finished product. Even when such is the case, however, a dramatic strength improvement has been revealed, and the product is reproducible.

A 200 gram charge of fine powders (325 mesh) of a columbium-5 weight percent zirconium alloy, analyzing at 200 p.p.m. carbon, was fluidized with purified argon at 2000 F. in apparatus of the type previously described. Methane, which dissociates at this temperature, was introduced into the inert gas stream at a volume flow rate approximating one-fortieth that of the argon, and the reaction was permitted to continue for 40 minutes.

The reacted powders were then held in a vacuum for 2 hours at 500 F. to effect outgassing, formed into a 1 inch diameter compact under a pressure of 20 tons/ inch, and canned with a columbium jacket of 1%; inch outside diameter. The canned compact was subsequently extruded at 2200 F., followed by a swaging operation at 1600 F. through a 0.968 inch die.

Post treatment metallurgical analysis indicated a total carbon content of 4400 p.p.m. and revealed that approximately 20 percent of the metal charge was reacted. The envelope comprised a double carbide case (Cb,Zr)C and (Cb,Zr) C, and imparted a significant high temperature strength improvement to the basic matrix metal.

The columbium-zirconium alloys were selected as the initial matrix metal both because of their availability and because of the proposed use for the finished product. Later investigations with various alloys were conducted to establish the wide applicability of the techniques described and to resolve the preferred operating parameters for the process. At temperature levels of 12002000 F. reaction times of from 10-24 hours were found necessary to achieve uniform formation of the desired surface envelope, depending on the alloy and reactant gas compositions and the reaction temperatures. In general, the lower melting point alloys such as titanium, and more specifically, titanium-5 weight percent aluminum are reacted in the temperature range of 1200l800 F. while the higher melting alloys of tantalum, tungsten, columbium and molybdenum are maintained at l400-2000 F. during the formation of the hard phase envelopes.

As previously indicated, particle size in the present process is not a particularly critical parameter. It is obvious, however, that dispersoid distribution is improved and powder reaction times are reduced as in inverse function of particle size. However, cold fabricability decreases as a function hard phase content of the reacted powders. It will generally be found preferable, therefore, to utilize powders of 200 to 325 mesh, and to adjust the reaction times to provide an envelope buildup of from 0.1 to 0.2 mil on the individual particles, although, of course, more or less is satisfactory as previously established.

From the foregoing it will be evident that in the present invention there has been provided a greatly improved method of preparing the dispersion strengthened alloys, the method being readily compatible with existing powder metallurgy techniques and apparatus and providing greatly improved reproducibility of product over existing alloy preparation methods.

While the instant invention has been described in considerable detail with respect to the more preferred techniques and materials, it will be seen to have broad appli-- cability to the metals industry. Accordingly, no limitation is intended by the detailed discussion within the spirit of the invention as set forth in the appended claims.

What is claimed is: 1. The method of preparing carbide dispersion strengthened alloy articles comprising the steps of:

forming fine powders of a matrix metal selected from the group consisting of columbium, tantalum, molybdenum, tungsten, titanium and alloys thereof, agitating the powders in a fluidized bed at 12002000 F. in an inert gas stream, introducing a carbonaceous ,gas into the inert gas stream, the carbonaceous gas thermally dissociating at the bed temperature and chemically reacting with the matrix metal and forming a thin carbide film on the individual powder particles, adjusting the flow rate of the carbonaceous gas to effect the desired carbide formation while preventing the accumulation of substantial amounts of unalloyed carbon in the film, and working the reacted powders to produce a wrought article in which the carbide is uniformly dispersed. 2. The method of claim 1 in which the carbonaceous gas is methane.

3. The method of claim 2 in which the volume flow of methane constitutes about two percent of the total gas flow through the bed.

4. The method of claim 2 in which the alloy is a columbium base alloy.

5. The method of claim 4 in which the reacted powders are worked at a temperature below their normal sintering temperature.

6. The method of claim 5 in which the reaction is continued until a carbide film thickness of at least 0.1 mil thickness has been formed on the powder particles.

References Cited UNITED STATES PATENTS 2,844,492 7/1958 Fitzer 75-202 3,073,698 1/1963 Arbiter 752l2 3,077,385 2/1963 Robb 117106 3,163,527 12/1964 Storchheim 752l2 BENJAMIN R. PADGETT, Primary Examiner.

R. L. GRUDZIECKI, Assistant Examiner. 

1. THE METHOD OF PREPARING CARBIDE DISPERSION STRENGTHENED ALLOY ARTICLES COMPRISING THE STEPS OF: FORMING FINE POWDERS OF A MATRIX METAL SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM, TANTALUM, MOLYBDENUM, TUNGSTEN, TITANIUM AND ALLOYS THEREOF, AGITATING THE POWDERS IN A FLUIDIZED BED AT 1200-2000* F. IN AN INERT GAS STREAM, INTRODUCING A CARBONACEOUS GAS INTO THE INERT GAS STREAM, THE CARBONACEOUS GAS THERMALLY DISSOCIATING AT THE BED TEMPERATURE AND CHEMICALLY REACTING WITH THE MATRIX METAL AND FORMING A THIN CARBIDE FILM ON THE INDIVIDUAL POWDER PARTICLES, ADJUSTING THE FLOW RATE OF THE CARBONACEOUS GAS TO EFFECT THE DESIRED CARBIDE FORMATION WHILE PREVENTING THE ACCUMULATION OF SUBSTANTIAL AMOUNTS OF UNALLOYED CARBON IN THE FILM, AND WORKING THE REACTED POWDERS TO PRODUCE A WROUGHT ARTICLE IN WHICH THE CARBIDE IS UNIFORMLY DISPERSED. 